Magnetic resonance imaging (MRI) allows images of tissue to be captured in vivo and in a non-invasive manner. MRI is unlike X-rays or CT scans in that MRI employs magnetic fields to produce an image, as opposed to ionizing radiation. Therefore, MRI is relatively harmless. Accordingly, MRI is now used in a range of applications in the field of medical diagnostics. In clinical MRI, a patient is placed within a scanner of an MRI system and a region of interest of the patient is exposed to a magnetic field generated by a scanner of the MRI system also referred to as a localizer. Basically, the frequency of the magnetic field is selected to resonate (i.e., excite) certain atoms making up the tissue in the region of interest, and emissions of the excited atoms are captured and analyzed to produce an image of the tissue.
Magnetic resonance spectroscopy (MRS), also known as nuclear magnetic resonance (NMR) spectroscopy, combines magnetic resonance imaging and spectroscopic techniques. Basically, in MRS, an MRI system is controlled to first perform an MRI scan that captures an image of an anatomical region of interest in a sample or patient, and the image is used to plan for the spectroscopy including for helping to identify a localized volume within the region of interest where the spectroscopy is to be carried out. One aspect of controlling the MRI system to perform the spectroscopy is to “shim” the magnetic field generated by the MRI system in an attempt to maximize the homogeneity of the field over the localized volume within the region of interest. A spectra or spectrum of emissions from the tissue confined to this localized volume within the shimmed magnetic field are captured and analyzed.
The spectrum/spectra acquired by MRS contains information about metabolites making up the tissue in the localized volume within the region of interest. Therefore, MRS is a technique of choice to acquire images and information of tumors, especially tumors such as those which may be present in the brain. Such information can be used to diagnose and stage the tumors.
Furthermore, MRS may be classified as single voxel MRS or multi-voxel MRS (also referred to as chemical shift imaging).
In single voxel MRS, as the name implies, an anatomical planning image of the region of interest is obtained through conventional MRI, and a single volume (the single voxel) is defined and located in the region of interest under the control of technician based on an observation of the anatomical planning image and his or her experience with the protocol of the suspected pathology. At this time, as described above, the magnetic field is shimmed for the single voxel. As a result, a spectrum of magnetic resonances from the single volume is obtained. Metabolite concentrations can be measured from the spectrum to yield information on characteristics of the tissue within the region of the single voxel.
In CSI, spectra of emissions are obtained from several volumes in a 2D slice of interest or a 3D region of interest. Thus, although CSI is advantageous over single voxel MRS in that a larger sample in a region of interest can be examined, it is disadvantageous in that it provides a lower signal-to-noise ratio (SNR) and requires a longer scan time.
However, the clinical success of using single voxel MRS depends strongly on not only the shimming of the magnetic field but the location where the single voxel is placed within the region of interest.
Specifically, placing the single voxel in an area of high magnetic field homogeneity can result in good single voxel results, while placing the single voxel in an area of poor field homogeneity, i.e., in an area of magnetic field inhomogeneity, can lead to a low signal-to-noise ratio (SNR). The process of single voxel placement can be operator dependent, leading to poor results especially when conducted by an untrained or relatively inexperienced MRI technician.